1. Field of the Invention
Embodiments of the present invention generally relate to methods for estimating efficiency and controlling the operation of a downhole pump. More particularly, embodiments of the present invention generally relate to methods for estimating efficiency and controlling the operation of a conventional sucker-rod pump.
2. Description of the Related Art
The production of oil with a sucker-rod pump such as that depicted in FIG. 1 is common practice in the oil and gas industry. The sucker-rod pump 100 is driven by a motor 110 that turns a crank arm 120. Attached to the crank arm 120 is a walking beam 130 and a Horsehead 140. A cable 150 hangs off the Horsehead 140 and is attached to a sucker-rod 155. The sucker-rod 155 is attached to a downhole pump 160 located within the wellbore 165. A portion of the sucker-rod 155 passes through a stuffing box 170 at the surface. That portion of the sucker-rod is called the polished rod 175. In operation, the motor 110 turns the crank arm 120 which reciprocates the walking beam 130 which reciprocates the sucker-rod 155.
The downhole pump 160 includes a barrel 180 that can be attached to or part of the production tubing 185 within the wellbore 165. A plunger 187 is attached to the end of the sucker-rod 155 and reciprocates in the barrel 180. The barrel 180 includes a standing valve 190. The plunger 187 is provided with a traveling valve 195. On the up stroke of the plunger 187, the traveling valve 195 closes and the fluid is lifted above the plunger 187 to the top of the well, and the standing valve 190 opens to allow additional fluid from the wellbore 165 into the barrel 180. On the down stroke of the plunger 187, the traveling valve 195 opens and the standing valve 190 closes, allowing the plunger 187 to pass through the fluid which is being held in the barrel 180 by the standing valve 190.
Typically, the pumping system is designed with the capacity to remove liquid from the wellbore 165 faster than the reservoir can supply liquid into the wellbore 165. As a result, the downhole pump does not completely fill with fluid on every stroke. The well is said to be “pumped-off” when the pump barrel 180 does not completely fill with fluid on the upstroke of the plunger 187. The term “pump fillage” is used to describe the percentage of the pump stroke which actually contains liquid.
Varying degrees of mechanical damage can occur to the pumping system if the pump is operated with substantially less than 100% pump fillage for extended periods of time (i.e. when the well is pumped-off). During pumped-off conditions, the plunger contacts the fluid in an incompletely filled barrel at which point the traveling valve will open. The impact between the plunger 187 and fluid known as “fluid pound” will cause a sudden shock to travel through the sucker-rod 155 and the pumping unit 100 which can cause damage to the sucker-rod 155 and other pumping components. Thus, an effort is made to shut down the pumping unit when the well reaches a pumped-off condition to prevent damage to the equipment as well as to save power.
Automation devices have been used with sucker-rod pumping systems to monitor and temporarily discontinue pumping operations to protect the pump. Surface dynamometer data have long been used as a basis for controlling sucker-rod pumping systems. Historically, measured operating characteristics of the pumping unit have been used to derive a data set representing load (force) on the polished rod vs. displacement of the polished rod (known as a “surface dynamometer card”). Various algorithms have subsequently been applied to these data sets to identify a “pump-off” condition.
However, the surface dynamometer card does not supply an accurate depiction of the operation of the downhole pump due to the elasticity of the sucker-rod string and viscous damping effects among other operating conditions. With longer sucker-rods and larger pump sizes (higher stress) and even revolutionary new sucker-rod materials, the differences between the displacement versus time at the surface and the displacement versus time at the downhole pump can be quite dramatic. Therefore, methods of controlling sucker rod pumping units based upon surface dynamometer cards can be prone to error. In addition, the elasticity of the sucker rod string causes the stroke length of the downhole pump to differ from the stroke length of the polished rod. This introduces further error into production volume estimates.
Therefore, measurements taken at the pump are more reliable and less prone to error. Since direct measurement of the load and displacement at the pump in the wellbore is cost prohibitive in most production operations, attempts have been made to mathematically model or infer “downhole dynamometer cards” (load vs displacement at the downhole pump) from the surface dynamometer card and other static data. Those models are capable of providing an approximation of the actual downhole dynamometer card. However, the execution of those models in a remote setting (i.e. at the well site) requires considerable computing capacity. Additional logic must also still be applied to make a pump-off determination once the downhole dynamometer card has been mathematically simulated. Furthermore, existing methods including downhole dynamometer cards provide no direct means of estimating pump fillage. As a result, still more computational effort is required to derive the information needed to support reliable estimates of pump production.
There is a need, therefore, for a method for determining pump fillage and a method for controlling pump operations without deriving a downhole dynamometer card.